Amperes Law states that the integral of the magnetic field around a closed loop surrounding a current source is equal to the current enclosed. This is the principle used in conventional one wire current probes and the method of measurement is shown in FIG. 1.
A magnetic material 10 completely surrounds a current carrying wire 12 and a coil 14 having a large number of turns of wire is wound uniformly on the magnetic material 10. The voltage induced in the coil 14 is directly proportional to the current I flowing in the wire 12 that the magnetic material 10 encloses.
This type of probe has the advantage that any other current carrying wire outside the probe induces no voltage in it and a relatively large voltage is induced in it from the single current carrying wire it surrounds.
However, it cannot readily be used to measure current in cables that have more than one current carrying wire, for example, a two or three wire cable, as the current that flows out in one wire returns in the other(s) and so the current enclosed is zero and no voltage will be induced in the coil 14. This would entail removing the outer layer of the cable so that an individual wire can be separated sufficiently to place the probe around it. In the vast majority of single phase wiring, however, two or three wire cables are used with no ready access to a single wire.
It is an object of the apparatus to provide an apparatus for measuring the current flowing in an electric cable where the cable has more than one current-carrying wire, for example oval or round cables having two wires or two wires and earth, and where currents nominally of equal amplitude flow in opposite directions along the cable.